1. Field of the Invention
The present invention relates to a method for evaluating the image quality of color liquid crystal displays, and more particularly, to a method for evaluating the image quality of color liquid crystal displays by measuring the modulation transfer function (MTF), and a system for measuring the MTF.
2. Description of the Related Art
Color liquid crystal displays (LCDs) have been widely used not only for computer displays but also for television displays, in place of conventional cathode ray tubes (CRTs) because of their portability. The resolution of LCDs is generally calculated from the modulation transfer function (MTF) formula with design data. In this formula, the luminance distribution of a dot is assumed to be a rectangular shape. Practically, however, the luminance distribution of a dot is not a perfect rectangular shape. Therefore, the following factors should be considered in evaluating the image quality of LCDs using the MTF formula: degradation of the image quality by unexpected spurious luminance patterns and chromatic noise, nonuniformity of the luminance distribution caused by optical characteristics, and defects introduced during production.
Usually, LCDs have contrast and brightness controls. Because the optimal image quality is determined by the contrast and brightness controls, there is a need to find how the combinations of the contrast and brightness levels influence the image quality.
Color LCDs emit a certain spectral distribution, which, together with the spectral response of the human eye, is associated with the luminance distribution. If a photodetector used to measure the luminance distribution is sensitive to color, there is a need to correct for a response difference between the human eye and the photodetector.
To solve the above problems, it is an objective of the present invention to provide a system for measuring the MTF for use in evaluating the image quality of color LCDs in consideration of parameters such as contrast and brightness, and a method for evaluating the image quality of color LCDs using the system.
According to an aspect of the present invention, there is provided a system for measuring an MTF for use in evaluating the image quality of color LCDs, the system comprising: an objective lens for magnifying a white dot on the screen of a color LCD to be tested; a charge coupled device (CCD) camera for capturing the luminance distribution of the white dot magnified by the objective lens; an analog-to-digital converter (ADC) for converting analog information of the luminance distribution of the white dot captured by the CCD camera to digital information; and a controller for correcting the magnification of the objective lens over the digital luminance distribution information of the white dot, and Fourier transforming the line spread function (LSF) according to the luminance distribution to measure the MTF.
The controller may be a computer.
Preferably, the CCD camera comprises a V(A) filter for modifying the spectrum response characteristics of the CCD camera to closely approximate those of the human eye.
Preferably, the controller measures the MTF according to signal-to-noise (S/N) ratio in consideration of the background luminance (BL) of an image.
According to another aspect of the present invention, there is provided a method for evaluating the image quality of color LCDs, using a modulation transfer function algorithm (MTFA) at a predetermined position on the screen of an LCD.